<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
  <input id="num1" />*<input id="num2" />
  <input type="button" value="=" onclick="jisuan()"/>
  <input id="num3" />
<script>
  function jisuan(){
         num3.value =num2.value*num1.value;
     }
</script>
</body>
</html>